1. Field of the Invention
This invention relates to drilling of wells. More particularly, improved apparatus and method for lifting blowout preventers below the floor of a drilling rig are disclosed.
2. Description of Related Art
As part of the process of drilling wells in the earth, joints of pipe called "casing" are joined and placed in a hole drilled to a first intermediate depth to form a casing "string." Cement is pumped between the casing string and the wall of the hole to mechanically hold the casing in place and prevent flow outside the casing. The well is then drilled deeper through this string of casing. Before drilling begins through the casing, blowout preventers are attached to the top of the casing. The purpose of the blowout preventers is to seal the top of the casing should excess pressure be encountered when drilling the well deeper.
In land drilling rigs, a blowout preventer stack extends from the top of the casing upward for about 10 to 20 feet above the ground surface or the casing. The floor of the drilling rig is elevated on a substructure sufficiently that the floor of the rig is above the top of the blowout preventer stack. This stack may include several blowout preventers, which are selected to achieve a seal at the top of the well under a variety of drilling conditions. When another string of casing is placed in the well through the first string and down to an intermediate depth or to the total depth of the well, it is necessary to remove the blowout preventer stack and cut the top of the additional casing near the earth's surface. A stack of blowout preventers may weigh more than 50 tons.
It is known to use a compact winch unit placed upon the floor of the drilling rig, specifically on the rotary table, which is placed in an opening of the rig floor, to lift blowout preventers. Such compact winch units are disclosed and claimed in U.S. Pat. No. 4,305,467 (the '467 patent). This patent describes the placement of two winches on a base frame, positioning the base frame on the rotary table of a rig and lifting blowout preventers by the winches. Cables (also called "wire ropes" or "ropes") are present on the drums of the winches. The cables are preferably sized to have sufficient strength to lift blowout preventers and maintain an adequate safety factor. The '467 patent states (Col. 12) "In order to minimize the danger of breakage of the cables 56, 58, such cables are selected to have a breaking strength considerably larger than the weight of a blowout preventer."
The breaking strength of cables or wire ropes depends on their diameter. For example, according to a brochure from Delta Wire Rope, steel core wire rope in the 6.times.19 class made from improved plow steel and having a diameter of 1 inch has a minimum breaking strength of 51.7 tons. For a diameter of 11/8 inches, the minimum breaking strength is 65 tons. Although higher breaking strength can be achieved by use of larger diameter cable, the size drum which must be used to avoid excessive bending of the cable also increases with diameter of the cable. If larger diameter drums are used, placement of the winches on the floor of the drilling rig becomes increasingly difficult or, in some cases, impossible. The minimum size drum for 11/8 inch wire rope, for example, is 14 inches. To increase the breaking strength of the wire rope from 65 tons to 132 tons, the size of the wire rope must be increased from 11/8 inch to 115/8 inch. This would require an increase in drum size from about 14 inches to about 26 inches, which is often not practical, because there is not enough room on the rig floor.
What is needed is apparatus to achieve a higher safety factor with wire ropes on winch apparatus used for lifting blowout preventers and which can be as compact as presently used winches. Procedure for adapting prior art apparatus and operating cables on winches to achieve the higher safety factor is also needed.